In motor vehicle transmissions having hydraulically-engaged clutches (or other friction devices), it is desirable to regulate the clutch pressure to a value that is sufficient to prevent clutch slippage without being unnecessarily high. This can be achieved with a hydraulic circuit including a hydraulic pressure regulator valve and an electrically-operated boost valve that supplies a bias or boost pressure to the regulator valve. The relationship between the boost pressure and the clutch pressure can be established by a way of a look-up table, for example, and once the desired clutch pressure is known, the transmission controller can use the look-up table to determine the corresponding boost pressure. In a typical mechanization, the boost valve receives input hydraulic fluid which is either directed to the pressure regulator valve or exhausted to a fluid reservoir. The exhaust of fluid flow within the valve is determined by the position of an armature with respect to a seat, and a solenoid coil surrounding the armature is variably activated to position the armature.
It has been found that very fine debris normally present in filtered hydraulic transmission fluid can accumulate in the boost valve and prevent proper valve operation. In particular, the debris can accumulate between the armature and the valve seat under conditions where the armature is positioned very close the seat. The accumulation of debris then prevents the armature from engaging the seat when conditions so require, causing the produced boost pressure, and therefore the clutch pressure, to deviate from the desired value. Depending on the design of the valve, the erroneous clutch pressure can result in clutch slippage and other harmful consequences. Accordingly, what is needed is a way of preventing erroneous operation of the boost valve due to accumulation of debris without noticeably affecting the operation of the transmission.